Slow download speed on Kubuntu

I guess I have messed up something in my Kubuntu 19,04 somehow. Newly installed a couple of weeks ago.
I have cable internet with 300/100Mbit line.
I first noticed that updates took long time to download. First I didnt care, I thought it was just the Kubuntu servers.
But then after a while I checked with speedtest.net and noticed I only get around 75Mbit down and up.
I dual booted to Windows 10 on another SSD, and made the test again, but then I got normal download and upload speed: around 280-290Mbit.
I then booted to the Kubuntu installation USB and the Live session, even there it gave normal download speed.
When I checked the connections settings in the Live USB and my installed Kubuntu, they are the same.
I do have OpenVPN installed (via CLI) for my VPN (Privatevpn) in Kubuntu, but it is the same speed whether I use the VPN or not.
Did the same with a Linux Mint Live USB and no problems there either, nor did I have any problem with deepin linux that I had installed before I installed Kubuntu. I only have the problem in my installed Kubuntu. Can it be the openVPN installation that is the trouble? But surely it shouldnt interfere with the wired connection if I don't connect with VPN?

could you check whether the default network adapter on kubuntu is a gigabit adapter? something might be limiting it to 100 mbps.

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Yes it is a gigabit adapter. But this is very strange. Last night I played with the "Allow auto-negotiation" in the wired connection setting, and the speed set to 1000Mbit when it was off. But that didnt help so I went back to default and put the computer to sleep. This morning when I woke up my self and the computer I get full speed..
It was nothing wrong with my connection since I got full speed in windows and the Live session of Kubuntu. Very strange.
But thanks for your effort, much appreciated.

Yes it is a gigabit adapter. But this is very strange. Last night I played with the "Allow auto-negotiation" in the wired connection setting, and the speed set to 1000Mbit when it was off. But that didnt help so I went back to default and put the computer to sleep. This morning when I woke up my self and the computer I get full speed..
It was nothing wrong with my connection since I got full speed in windows and the Live session of Kubuntu. Very strange.
But thanks for your effort, much appreciated.